Ultrasound generally refers to sound waves that have a frequency above the range of human hearing. In a typical ultrasonic imaging system, short bursts of ultrasonic energy are directed into a body with a hand-held transducer. The returning reflected energy or echoes are received with the same transducer. The signals representing the reflected energy are processed and formatted into a video image of the target region. Ultrasonic imaging is used widely in medical applications to non-invasively observe structures within the human body, such as cardiac structures, the vascular system, the fetus, the uterus, the abdominal organs and the eye.
The ultrasonic transducer typically comprises an array of transducer elements, such as piezoelectric crystals, which convert electric signals to acoustic energy sufficient to penetrate the various structures in the human body. The transducer elements also convert the relatively weak returning echoes into electric signals which are processed into an image. The construction and function of an ultrasonic transducer array are well known in the art.
An electronic scanning subsystem controls the transmission and reception of ultrasound signals by the transducer elements in the array. The construction and theory of operation of such a scanner for an ultrasonic imaging system are outlined in an article entitled "Electronic Scanner for Phased-Array Ultrasound Transducer" Gatzke et al., Hewlett Packard Journal, December 1983, which is hereby incorporated by reference. The scanner controls the steering and focusing of the ultrasonic beam. The scanner includes, in the receiving path from each transducer element, a time gain compensation (TGC) amplifier. Echoes from structures near the transducer array are relatively large in amplitude, whereas echoes from structures deep within the body, received later, are relatively small in amplitude. The TGC amplifier compensates for the wide range in received amplitudes by providing a gain that varies with time as the ultrasonic echoes are being received.
The range of signals produced by the transducer element array may be larger than the range of signals that can be received by the receiving circuitry within the scanner subsystem. Accordingly, the noise performance of the receiving amplifier within the scanner becomes critical to the overall noise performance of the system. Also, the dynamic range of the system will be limited by the ability of the receiving circuitry within the scanner to handle very high signal levels.
A hand-held probe containing the transducer array is typically connected to the receiving and processing electronics of the system through a connecting cable. This arrangement can cause impedance matching problems. If the impedance of each transducer element is not matched to the impedance of the cable, waveform distortion can result. For modern acoustic transducer elements, the transducer element impedance is typically much higher than the impedance of the cable.
One solution to the impedance matching problem is to provide a fixed gain preamplifier after each transducer element to drive the cable. U.S. Pat. No. 4,489,729 issued Dec. 25, 1984 to Sorenson et al discloses a fixed gain linear preamp mounted in the scanner head of an ultrasound imaging system. A limitation with this approach is the amount of gain which can be employed in the preamplifier. If the gain is low (unity gain is typical), the noise performance of the system receiving amplifier is critical to the overall noise performance of the imaging system. Further, noise degradation can result from external electromagnetic interference (EMI) sources due to the relatively low signal levels produced by the preamplifier. If the gain of the preamplifier is high, the dynamic range of the system may be limited by the inability of system receiver to handle high signal levels.